Recording medium holding apparatus, image forming apparatus, and image forming method

ABSTRACT

According to one embodiment, an image forming apparatus includes an endless device, a power supply unit, an electric charge supplementary unit, and an image forming unit. The endless device is rotated. The power supply unit supplies a recording medium on a surface of the endless device with an electric charge. The electric charge supplementary unit supplements the electric charge to the recording medium which is rotated together with the endless device in the state of being attracted to the surface of the endless device by the electric charge provided by the power supply unit. The image forming unit forms an image on the recording medium attracted to the surface of the endless device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromU.S. Provisional Application No. 61/359,160, filed on Jun. 28, 2010; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a recording mediumholding apparatus, an image forming apparatus, and an image formingmethod.

BACKGROUND

In related art, there is known an image forming apparatus having amechanism that holds a recording medium such as a sheet on the surfaceof a drum as a holding member by an electrostatic force or a negativepressure. For example, an image forming apparatus such as an ink jetprinter forms an image on a recording medium in an attracted state onthe surface of the drum. The type of image forming apparatus requiresthat the recording medium is reliably attracted to the surface of thedrum so as to form a stable image on the recording medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that schematically shows a configuration example ofan image forming apparatus.

FIG. 2 is a diagram that shows a configuration example of a controlsystem in the image forming apparatus.

FIG. 3 is a diagram that shows an example of electric potentialfluctuation in the sheet attracted to the surface of the drum.

FIG. 4 is an example of a table in which various printing conditionscorrespond to correction voltage values as electric charge supplementaryamounts.

FIG. 5 is a flow chart for explaining an example of an image formingprocessing.

DETAILED DESCRIPTION

In general, according to one embodiment, an image forming apparatusincludes an endless device, a power supply unit, an electric chargesupplementary unit, and an image forming unit. The endless devicerotates. The power supply unit supplies the recording medium on thesurface of the endless device with an electric charge. The electriccharge supplementary unit supplements the electric charge to therecording medium rotated together with the endless device in a state inwhich the recording medium is attracted to the surface of the endlessdevice by the electric charge provided by the power supply unit. Theimage forming unit forms the image on the recording medium attracted tothe surface of the endless device.

Hereinafter, an exemplary embodiment will be described with reference tothe drawings in detail.

The image forming apparatus according to the present embodiment includesa recording medium holding apparatus that holds a sheet as a recordingmedium. The image forming apparatus according to the present embodimentforms an image on the recording medium held by the recording mediumholding apparatus. The present embodiment can be applied to an imageforming apparatus including a mechanism that holds the recording medium.For example, the present embodiment can be applied to various imageforming type printers such as an ink jet type printer, an electronicphotographic type printer, and a heat transfer type printer.

FIG. 1 is a diagram that schematically shows a configuration example ofan image forming apparatus 1 according to the present embodiment.

The image forming apparatus 1 shown in FIG. 1 is an ink jet type printer(an ink jet printer).

As shown in FIG. 1, the image forming apparatus 1 has a sheet feedingunit 11, a sheet holding unit 12, an image forming unit 13, a sheetdischarging unit 14, and a reversing unit 15. The sheet feeding unit 11feeds a sheet as a recording medium to the sheet holding unit 12. Thesheet holding unit 12 holds the fed sheet as the recording medium in astate in which an image can be formed by the image forming unit 13. Theimage forming unit 13 forms the image on the sheet held by the sheetholding unit 12. The sheet discharging unit 14 discharges the sheet onwhich the image is formed by the image forming unit 13.

The sheet feeding unit 11 has a sheet feeding cassette 21, a pickuproller 22, a sheet feeding roller 23, a separation roller 24, a sheettransport path 25, and a resist roller pair 26. The sheet feedingcassette 21 receives a plurality of sheets. The pickup roller 22 takesthe sheet out of the sheet feeding cassette 21. The sheet feeding roller23 and the separation roller 24 separate one sheet of the sheet takenout by the pickup roller 22. The sheet feeding roller 23 and theseparation roller 24 sends the sheet P, which is one separated sheet, tothe sheet transport path 25. The sheet transport path 25 guides thesheet P transported by the sheet feeding roller 23 and the separationroller 24 to the resist roller pair 26.

The resist roller pair 26 delivers the sheet P at a desired timing. Theresist roller pair 26 corrects oblique motion of the sheet P. That is,the resist roller pair 26 corrects the oblique motion of the sheet P anddelivers the sheet P, in which the oblique motion is corrected, to adrum 31 as a holding member of the recording medium in the sheet holdingunit 12 at a desired timing. The drum 31 is an endless device in therotation direction.

The sheet holding unit 12 includes a function as a recording mediumholding apparatus. The sheet holding unit 12 has a drum 31, a chargingroller 33, a peeling charger 34, a peeling member 35, a cleaning member36, and a neutralizing charger 37. In the sheet holding unit 12, thecharging roller 33, the peeling charger 34, the peeling member 35, thecleaning member 36, and the neutralizing charger 37 are sequentiallyprovided at positions facing the surface of the drum 31 as the holdingmember of the sheet P.

The drum 31 is a holding member that holds the sheet P as the recordingmedium. In the configuration shown in FIG. 1, the drum 31 has acylindrical shape. The drum 31 is rotated in a direction a shown in FIG.1 around the cylindrical axis. In addition, the drum 31 may be a drum inwhich the cross section thereof perpendicular to the rotation directionis flat. The drum 31 has a cylindrical frame 31 a formed of a conductor,and a dielectric material 31 b on the surface of the frame 31 a.

For example, in the drum 31 in a configuration example shown in FIG. 1,the frame 31 a is formed of, for example, a metallic conductor such asaluminum. The dielectric material 31 b is formed on the surface of thecylindrical frame 31 a formed of a conductor as a thin layer. The frame31 a sets the electric charge to 0 [V] by grounding the inside. Theinside of the frame 31 a is a counter electrode relative to the chargingroller 33, and the electric charge thereof is held at 0 [V].

The charging roller 33 has a metal core 33 a along the axis of therotation shaft, and a cylindrical conduction member 33 b providedoutside the metal core 33 a. The charging roller 33 is rotated dependingon the rotation of the drum 31 in the state where the cylindricalconduction member 33 b is in contact with the surface of drum 31. Thecharging roller 33 is pressed against the surface of the drum 31. Asheet P is supplied from the resist roller pair 26 to a nip between thecharging roller 33 and the drum 31.

A voltage application circuit 33 c applying the voltage is connected tothe metal core 33 a. Before the sheet P enters to the nip between thecharging roller 33 and the drum 31, the voltage application circuit 33applies a high voltage to the metal core 33 a of the charging roller 33.Since the inside of the drum 31 is grounded, an electric potentialdifference depending on the electric potential applied to the chargingroller 33 is generated in the nip between the charging roller 33 and thedrum 31. That is, the charging roller 33 supplies the electric charge tothe sheet P, which is in close contact with the surface of the drum 31in the nip between the charging roller 33 and the drum 31, by thevoltage applied to the metal core 33 a.

The sheet P supplied with the electric charge by the charging roller 33is attracted to the surface of the drum 31 by electrostatic force. Thedrum 31 is rotated in the state of attracting (holding) the sheet P onthe surface thereof. The sheet P attracted to the surface of the drum 31is moved to the image forming position by the image forming unit 13along with the rotation of the drum 31.

The image forming unit 13 has a printing head group 41 that includes aplurality of printing heads 41C, 41M, 41Y, and 41K. The respectiveprinting heads 41 are provided to face the surface of the drum 31.

The printing head 41C discharges cyan ink. The printing head 41Mdischarges magenta ink. The printing head 41Y discharges yellow ink. Theprinting head 41K discharges black ink.

The respective heads 41 form the image on the surface of the sheet Pattracted to the surface of the drum 31 by discharging ink based onimage information. Furthermore, the drum 31 is rotated by number ofrevolutions depending on a printing condition in the state of attractingthe sheet P on the surface thereof. The drum 31 can move the sheet P ina state of being attracted to the surface to the image forming positionby the number of revolutions of the drum 31 in the multi pass. In otherwords, the printing head 41 of the image forming unit 13 can repeatedlyexecute image formation (hereinafter, referred as a multi-pass imageforming process) on the sheet P by the number of revolutions of the drum31.

For example, in the case of a configuration in which one color image isformed per rotation of the drum 31, by forming images using four colorsink (cyan, magenta, yellow, and black) constituting full color on asheet rotated together with the drum 31 four times, respectively, a fullcolor image can be finished. Furthermore, when forming the image of aresolution higher than the physical resolution of the printing head 41itself, by repeatedly forming the images on the sheet rotated with thedrum 31 several times, a high resolution image can be formed.Furthermore, when forming the image at a high density, by repeatedlyforming the images on a sheet rotated several times with the drum 31,the high density image can be formed. In addition, in the presentembodiment, a situation is contemplated where, after attracting thesheet P onto the drum 31, the drum 31 is rotated several times to formthe image on one surface of the sheet.

When the printing head 41 rotates the sheet attracted to the drum 31several times to execute several printings, until the image is finishedon the surface of the sheet, each of the peeling member 35, the cleaningmember 36, and the charging roller 33 may be separated from the surfaceof the drum 31 by the respective separation mechanisms. That is, afterthe sheet P is attracted to the surface of the drum 31, until the imageis completed, the peeling member 35, the cleaning member 36, and thecharging roller 33 are separated from the surface of the drum 31,whereby it is possible to prevent the peeling member 35, the cleaningmember 36, and the charging roller 33 coming into contact with thesurface of the sheet P until the image is completed.

The sheet P with the image formed by the image forming unit 13 is peeledoff from the surface of the drum 31 after being neutralized, and isdelivered to the transport roller 51. When the image formation on thesurface of the sheet P by the image forming unit 13 is finished, thepeeling charger 34 neutralizes the sheet P, and the peeling member 35peels off the sheet P from the surface of the drum 31.

The peeling charger 34 supplements the electric charge to the sheet Pattracted to the surface of the drum 31 or removes the electric chargefrom the sheet P attracted to the surface of the drum 31. The peelingcharger 34 supplies electricity or neutralizes the surface of the drum31 and the sheet P attracted to the drum in a non-contact manner. Forexample, when removing the electric charge from the sheet P (that is,when peeling off the sheet P from the drum 31), the peeling charger 34removes the electric charge from the sheet P by supplying the sheet Pattracted to the surface of the drum 31 with AC electric potential, anddissolves the electrostatic adhesion force to the drum 31. Furthermore,when supplementing the electric charge (supplying electricity) to thesheet P, the peeling charger 34 supplies (supplements) the electriccharge to the sheet P by supplying the sheet P attracted to the surfaceof the drum 31 with a desired electric potential difference. As aresult, the electrostatic adhesion force between the sheet P and thesurface of the drum 31 is reinforced.

The peeling member 35 peels off the sheet P from the surface of the drum31. Before the tip of the sheet P to be peeled off reaches a peelingposition, the peeling member 35 comes into contact with the surface ofthe drum 31. The peeling member 35 separates the sheet P, in which theimage formation is finished and which is neutralized by the peelingcharger, from the surface of the drum 31, in the state of being incontact with the surface of the drum 31. Furthermore, the peeling member35 guides the sheet P peeled off from the surface of the drum 31 to thetransport roller 51.

Moreover, between the peeling member 35 and the charging roller 33, thecleaning member 36 and the neutralizing charger 37 are provided. Thecleaning member 36 cleans the surface of the drum 31. The cleaningmember 36 is pressed against the surface of the drum 31 after peelingthe sheet P. The drum 31 is rotated in the state where the cleaningmember 36 is pressed against the surface of the drum 31, whereby thecleaning member 36 cleans the surface of the drum 31.

Furthermore, the neutralizing charger 37 neutralizes the drum 31 orsupplements the electric charge to the sheet P attracted to the surfaceof the drum 31. The neutralizing charger 37 supplies electricity orneutralizes the surface of the drum 31 or the sheet P attracted to thedrum in a non-contact manner. For example, when the drum 31 isneutralized (for example, after the sheet P is peeled off by the peelingmember 35), the neutralizing charger 37 neutralizes the drum 31.Furthermore, when supplementing (supplying electricity) the electriccharge to the sheet P, the neutralizing charger 37 supplies(supplements) the electric charge to the sheet P by supplying the sheetP attracted to the surface of the drum 31 with a desired electricpotential difference. As a result, the electrostatic adhesion forcebetween the sheet P and the surface of the drum 31 is reinforced.

The sheet discharging unit 14 has a plurality of transport rollers 51,52, and 53, a sheet discharge table 54. The sheet discharging unit 14transports the sheet P peeled off from the surface of the drum 31 by thepeeling member 35 to the sheet discharge table 54 by the transportrollers 51, 52, and 53. The transport roller 51 delivers the sheet Psupplied from the peeling position of the drum 31 to the transportroller 52. The transport roller 52 delivers the sheet P to be suppliedfrom the transport roller 51 to the transport roller 53. The transportroller 53 discharges the sheet P to be supplied from the transportroller 52 to the sheet discharge table 54.

Next, a configuration of a control system in the image forming apparatus1 will be described.

FIG. 2 is a diagram that shows a configuration example of a controlsystem in the image forming apparatus 1.

As shown in FIG. 2, the image forming device 1 has a control unit 70.The control unit 70 controls each unit of the image forming apparatus 1.The control unit 70 has a main control unit 71, a unit control circuit72, an internal interface 73, and an external interface 74.

The main control unit 71 has a CPU 81, a ROM 82, and a RAM 83. The CPU81 is a processor that executes the control of each unit, the dataprocessing or the like. The ROM 82 stores a control program, controldata or the like. The RAM 83 temporarily houses the data. In the maincontrol unit 71, the CPU 81 realizes various controls and dataprocessing by executing the program stored in the ROM 82 using the RAM83 as a working memory. Furthermore, the ROM 82 stores settinginformation for determining the electric potential for supplementing theelectric charge to the rotating sheet P attracted to the drum 31. Forexample, in the ROM 82, a table 82 a is stored which shows the magnitude(the supplementary amount of the electric charge to the sheet) of thecorrection voltage value depending on various printing conditions.

The unit control circuit 72 is a control circuit for operating each unitdepending on the control command from the main control unit 71. Theinternal interface 73 is an interface for connecting the unit controlcircuit 72 to each unit in the image forming apparatus 1. Furthermore,the external interface 74 is an interface for connecting an externalapparatus such as a host computer to the main control unit 71.

For example, the internal interface 73 is connected to a transportdriving mechanism 75, the printing head 41, the voltage applicationcircuit 33 c, the neutralizing charger 37, and the peeling charger 34.The transport driving mechanism 75 is a mechanism that drives thetransport rollers 22 to 24, 26, and 51 to 53, the drum 31 or the like soas to transport the sheet. For example, the transport driving mechanism75 is a motor that rotates the transport rollers 22 to 24, 26, and 51 to53, and the drum 31. The transport driving mechanism 75 operates thetransport rollers 22 to 24, 26, and 51 to 53, and the drum 31 dependingon the driving command from the main control unit 71.

The voltage application circuit 33 c is a circuit that applies thevoltage to the metal core 33 a of the charging roller 33. The voltageapplication circuit 33 c applies the voltage to the metal core 33 a soas to obtain the electric potential designated from the main controlunit 71. For example, the voltage application circuit 33 c applies thevoltage so that the sheet P (the first pass sheet) supplied from theresist roller pair 26 becomes the electric potential designated from themain control unit 71.

Furthermore, the voltage application circuit 33 c may apply the voltageto the charging roller 33 so as to provide the sheet P after the secondpass with the electric potential difference designated from the maincontrol unit 71. Furthermore, when the sheet attracted to the surface ofthe drum 31 passes more than second times (two passes), the chargingroller 33 may not come into contact with the surface of the drum 31 andthe sheet P on the drum 31. Even if the charging roller 33 does notcontact, it is possible to supply (supplement) the electric charge tothe sheet P on the drum 31 by the voltage provided to the metal core 33a by the voltage application circuit 33 c.

Furthermore, the peeling charger 34 neutralizes the sheet P attracted tothe surface of the drum 31. Furthermore, the peeling charger 34 maysupplement the electric charge to the sheet P attracted to the surfaceof the drum 31. In this situation, the peeling charger 34 applies thevoltage to the sheet P so as to obtain the electric potential designatedfrom the main control unit 71. Furthermore, the peeling charger 34neutralizes or supplies electricity without coming into contact with thesurface of the drum 31 and the sheet P attracted to the drum 31.

Furthermore, neutralizing charger 37 has a function of neutralizing thesheet P attracted to the surface of the drum 31 and a function ofsupplementing the electric charge to the sheet P attracted to thesurface of the drum 31. For example, the neutralizing charger 37 appliesthe voltage to the sheet P so as to obtain the electric potentialdesignated from the main control unit 71. Furthermore, the neutralizingcharger 37 supplies electricity or neutralizes without coming intocontact with the surface of the drum 31 and the sheet P attracted to thesurface of the drum 31.

In addition, the supplement of the electric charge to the sheetattracted to the drum 31 may be executed by at least any one of theneutralizing charger 37, the peeling charger 34 or the charging roller33. In the image forming apparatus of the present embodiment, any one ofthe neutralizing charger 37 or the peeling charger 34 supplements theelectric charge to the sheet (the sheet after second pass) P attractedto the drum 31.

Furthermore, the operation panel 90 is connected to the main controlunit 71. The operation panel 90 is a user interface by which a userinputs the operation instruction such as a printing condition. Forexample, the operation panel 90 is constituted by a display apparatusequipped with a hard key and a touch panel. The type (for example, size,thickness or the like) of sheet P, the humidity in the apparatus (or theperiphery of the apparatus), the state (for example, printing rates,resolution, color or the like) of the image to be formed on the sheet orthe like as printing conditions are input to the operation panel 90.

Next, the electric potential of the sheet P attracted to the surface ofthe drum 31 will be described.

FIG. 4 is a diagram that shows an example of the electric potentialvariable in the sheet P attracted to the surface of the drum 31.

When the sheet P is supplied from the resist roller pair 26 to the nipbetween the drum 31 and the charging roller 33, the charging roller 33makes the sheet P a predetermined electric potential V1 by the voltageto be applied from the voltage application circuit 33 c to the metalcore 33 a. The inner portion of the drum 31 is grounded. For thisreason, in the nip between the drum 31 and the charging roller 33, thevoltage of the electric potential difference V1 is applied to the sheetP of the surface of the drum 31 and the electric charge is supplied. Thesheet P provided with the electric charge by the electric potentialdifference V1 provided from the charging roller 33 is attracted to thesurface of the drum 31 by the electrostatic force. The drum 31 isrotated by the number of revolutions required for the image formationwhile attracting the sheet P.

When the drum 31 is rotated first, the sheet P is attracted to thesurface of the drum 31 by the electrostatic force due to the electriccharge provided by the electric potential difference V1. The electriccharge supplied to the sheet P gradually leaks owing to several factors.When the electric charge leaks from the sheet P, the electrostaticadhesion force onto the surface of the drum 31 is weakened. The sheet Phaving the weakened electrostatic adhesion force is easily separatedfrom the surface of the drum 31. That is, the sheet attracted to thedrum 31 is subjected to the leakage of the electric charge as timepasses, and the electrostatic adhesion force onto the surface of therotating drum 31 declines.

When the sheet P floats from the surface of the drum 31, the dischargingbased on Paschen's Law is generated, and the electric charge amount ofthe sheet P is attenuated. For example, a curl may be generated in thesheet due to moisture absorption or the like. In the curled sheet P, therepulsive elastic force overcomes the adhesion force onto the surface ofthe drum 31 due to the electrostatic force, and thus the curled sheet Pmay float from the drum surface. Furthermore, in the sheet having strongelastic force (for example, a thick sheet or a hard sheet), therepulsive elastic force overcomes the adhesion force onto the surface ofthe drum 31 due to the electrostatic force, and thus the sheet may floatfrom the drum surface.

Furthermore, generally, on the surface of the drum 31, the end portions(a tip or a rear) of the sheet P easily float compared to the centerportion thereof. Thus, in fact, the electric potential easily declinesin the end portion of the sheet P. In other words, the end portions ofthe sheet P is reliably attracted to the surface of the drum 31, and itis possible to suppress a decline in the electric charge amount in thesheet, which can stabilize adhesion onto the drum 31.

The image forming apparatus of the present embodiment supplements theelectric charge to the sheet P depending on the rotation of the drum 31in the image forming processing of multi-pass so as to cope with theleak of the electric charge as mentioned above. In the example shown inFIG. 3, in the sheet P, the electric potential difference V1 is providedin the first pass (when the sheet is supplied), the electric potentialdifference V2 is provided in the second pass, the electric potentialdifference V3 is provided in the third pass, and the electric potentialdifference V4 is provided in the fourth pass. Herein, the relationshipis V1≦V2≦V3≦V4.

That is, after the image formation (a first printing by the head 41) ofthe first pass is finished, until the image formation (a second printingby the head 41) of the second pass is started, the neutralizing charger37 (or the peeling charger 34, the charging roller 33) provides thesheet P attracted to the drum 31 with the electric potential differenceV2 to supplement the electric charge. In the example shown in FIG. 3, byproviding the electric potential difference V2, it is possible toexecute the processing of the second pass in the state of supplementingthe electric charge leaked during processing of the first pass.

Furthermore, after the image formation (the second printing by the head41) of the second pass is finished, until the image formation (the thirdprinting by the head 41) of the third pass is started, the neutralizingcharger 37 (or the peeling charger 34, the charging roller 33) providesthe sheet P attracted to the drum 31 with the electric potentialdifference V3 to supplement the electric charge. In the example shown inFIG. 3, by providing the electric potential difference V3, it ispossible to execute the processing of the third pass in the state ofsupplementing the electric charge leaked during processing of the secondpass.

Furthermore, after the image formation (the third printing by the head41) of the third pass is finished, until the image formation (the fourthprinting by the head 41) of the fourth pass is started, the neutralizingcharger 37 (or the peeling charger 34, the charging roller 33) providesthe sheet P attracted to the drum 31 with the electric potentialdifference V4 to supplement the electric charge. In the example shown inFIG. 3, by providing the electric potential difference V4, it ispossible to execute the processing of the fourth pass in the state ofsupplementing the electric charge leaked during processing of the thirdpass.

Next, a setting example of the electric charge amount to be supplementedto the sheet will be described.

FIG. 4 is a diagram that shows a setting example of a correction voltagevalue as the electric charge amount that is supplemented to the sheetdepending on the sheet state or the printing condition.

For example, a table 82 a shown in FIG. 4 stores the setting informationin which, the higher the printing rate of the image to be formed on thesheet is, the greater the electric potential (the correction voltagevalue) for supplementing the electric charge is. For example, the CPU 81acquires the printing rate designated by the operation panel 90 as theprinting condition. The CPU 81 sets the electric potential forsupplementing the electric charge depending on the acquired printingrate based on the setting information stored in the table 82 a.

The table 82 a shown in FIG. 4 stores the setting information in which,the higher the resolution of the image to be formed on the sheet is, thegreater the electric potential (the correction voltage value) forsupplementing the electric charge is. For example, the CPU 81 acquiresthe resolution of the image designated by the operation panel 90 as theprinting condition. The CPU 81 sets the electric potential forsupplementing the electric charge depending on the acquired resolutionbased on the setting information stored in the table 82 a.

Furthermore, the table 82 a shown in FIG. 4 stores the settinginformation in which the electric potential (the correction voltagevalue) for supplementing the electric charge is increased for the colorof the image to be formed on the sheet in order of yellow (Y), cyan (C),magenta (M), and black (K). For example, according to the table 82 ashown in FIG. 4, when the image to be formed on the sheet is color, theCPU 81 sets the electric potential for supplementing the electric chargeto a greater value than when the image to be formed on the sheet ismonochrome.

The table 82 a shown in FIG. 4 stores the setting information in which,the faster the transport speed of the sheet P is, the smaller theelectric potential (the correction voltage value) for supplementing theelectric charge is. For example, the CPU 81 determines the transportspeed depending on the printing condition input by the operation panel90. The CPU 81 sets the electric potential for supplementing theelectric charge depending on the transport speed based on the settinginformation stored in the table 82 a.

The table 82 a shown in FIG. 4 stores the setting information in which,the greater the thickness of the sheet P is, the greater the electricpotential (the correction voltage value) for supplementing the electriccharge is. For example, the CPU 81 determines the thickness of the sheetP by the type (for example, the sheet feeding cassette) of the sheet tobe selected by the operation panel 90. The CPU 81 sets the electricpotential for supplementing the electric charge depending on thethickness of the sheet based on the setting information stored in thetable 82 a. In addition, in the image forming apparatus 1, a thicknessdetection sensor may be provided which detects the thickness of thesheet P after feeding the sheet and until reaching the resist rollerpair 26. In this situation, the CPU 81 determines the electric potentialfor the electric charge supplement for multiple passes based on thethickness information detected by the thickness detection sensor.

The table 82 a shown in FIG. 4 stores the setting information in which,the higher the humidity (in the apparatus or periphery of the apparatus)is, the greater the electric potential (the correction voltage value)for supplementing the electric charge is. For example, the CPU 81determines the humidity by the humidity information input by a user bythe operation panel 90. The CPU 81 sets the electric potential for theelectric charge supplement depending on the humidity based on thesetting information stored in the table 82 a. In addition, in the imageforming apparatus 1, a humidity sensor may be provided which measuresthe humidity in the apparatus or the periphery of the apparatus. In thissituation, the CPU 81 determines the electric potential for the electriccharge supplement for multiple passes based on the humidity detected bythe humidity sensor.

The setting information stored in the table 82 a as mentioned above is,for example, set depending on the discharge property of ink or the likein advance. For example, in general, in many cases, a water-based inkhas a property of being easily discharged. Furthermore, each color ofink differs in composition, respectively. For this reason, the dischargeproperties differ even according to the color of ink. The table 82 a isset in view of the discharge property of ink.

Next, the image forming processing in the image forming apparatus 1 willbe described.

FIG. 5 is a flow chart for explaining an example of the image formingprocessing in the image forming apparatus 1.

When a user, who sets the printing condition in the operation panel 90,instructs the printing start, the CPU 81 of the main control unit 71acquires the printing condition instructed from the operation panel 90(ACT 11). When acquiring the printing condition, the CPU 81 feeds thesheet meeting the printing condition using the sheet feeding unit 11,and transports the sheet to the resist roller pair 26 (ACT 12). The CPU81 feeds the sheet P and initializes a variable n (n=1) (ACT 13). Inaddition, the CPU 81 determines the electric potential difference V1 foradhering the fed sheet onto the drum (ACT 14). The CPU 81, for example,determines the electric potential difference V1 depending on theprinting condition based on the table 82 a. Furthermore, the electricpotential difference V1 may be a preset fixed value.

When determining the electric potential difference V1, the CPU 81applies the voltage V1 to the metal core 33 a of the charging roller 33by the voltage application circuit 33 c before the sheet P is suppliedto the nip between the surface of the drum 31 and the charging roller 33(ACT 15). Since the inner portion of the drum is grounded, in the nipbetween the surface of the drum 31 and the charging roller 33, theelectric potential difference V1 is applied to the sheet P.

On the surface of the drum 31, the sheet P is attracted which isprovided with the electric charge by the electric potential differenceV1 in the nip between the surface of the drum 31 and the charging roller33 (ACT 16). The drum 31 is rotated in the state in which the sheet P isattracted to the surface. The sheet P attracted to the surface of thedrum 31 along with the rotation of the drum 31 to the image formingposition by the printing head 41. When the sheet P initially reaches theimage forming position (the first pass), the image forming unit 13executes the image formation (the first image forming processing) of thefirst on the sheet by the printing head 41 (ACT 17).

Whenever the sheet attracted to the drum 31 passes through the imageforming position by the image forming unit 13, the CPU 81 decideswhether or not the image is completed on the printing surface of thesheet P (ACT 18). For example, when the printing head 41 repeats theimage forming processing four times, whereby the image on the printingsurface of the sheet is completed, the CPU 81 decides whether or not theimage is completed depending on whether or not the variable n is “4”.

When it is decided that the image is not completed (ACT 18, NO), the CPU81 increments the variable n (n=n+1) (ACT 19) and decides whether or notthe electric charge needs to be supplemented to the sheet of n-throtation (ACT 20). When it is decided that the electric charge needs tobe supplemented to the sheet of the n-th rotation (ACT 20, YES), the CPU81 determines the electric potential difference Vn that applies theelectric charge relative to the sheet of n-th rotation as thesupplement, based on the table 82 a or the like (ACT 21). For example,the electric potential difference Vn is a value that meets therelationship of V (n−1)≦Vn. This is because the ink amount printed onthe sheet is increased whenever the number of passes is increased.Assuming that the greater the amount of ink on the sheet due to anincrease in the number of passes is, the easier the electric charge isdischarged, the CPU 81 determines the electric potential difference Vnso as to become a value satisfying V (n−1)≦Vn.

Upon determining the electric potential difference Vn, the CPU 81determines the position where the electric charge is supplemented in thesheet (ACT 22). For example, the CPU 81 selects any of the tip, therear, the tip and the rear, or the entire sheet as a position where theelectric charge is supplemented. For example, from the viewpoint ofpreventing that the end portion of the sheet P floats from the surfaceof the drum 31, the CPU 81 may determine the tip and the rear as thesupplementation position of the electric charge. Furthermore, the CPU 81may determine the supplementation position of the electric charge by theprinting condition such as the type of sheet (the thickness and thematerial) or the like.

When the supplementation position of the electric charge is determined,the CPU 81 applies the electric potential difference Vn to thesupplementation position of the electric charge in the sheet P attractedto the surface of the drum 31 by the neutralizing charger 37, therebysupplementing the electric charge (ACT 23). The neutralizing charger 37applies the electric potential difference instructed from the CPU 81 tothe sheet P in the state of not coming into contact with the sheet onthe drum 31. Furthermore, the supplement of the electric charge to thesheet on the drum 31 may be executed by the peeling charger 34 in anon-contact manner. Furthermore, the supplement of the electric chargeto the sheet on the drum 31 may be executed by the charging roller 33.When supplementing the electric charge by the charging roller 33, thecharging roller 33 may be separated from the sheet on the drum 31.

When supplementing the electric charge to the sheet P by the electricpotential difference Vn, the drum 31 of the n-th rotation moves thesheet P to the image forming position so as to perform the n-th imageforming processing to the sheet P. The image forming unit 13 performsthe n-th image forming processing (the image forming processing of then-th pass) to the sheet P on the drum 31 of the n-th rotation (ACT 17).

When the image forming processing of n-th pass is finished, the CPU 81decides whether or not the image is completed in the sheet P (ACT 18).When it is decided that the image on the sheet P is not completed (ACT18, NO), the CPU 81 repeatedly executes the processing of the ACTs 19 to23, and ACTs 17 and 18.

Furthermore, when it is decided that the image is completed in the sheet(ACT 18, YES), the CPU 81 neutralizes the sheet P, on which the image iscompleted, attracted to the surface of the drum 31, by the peelingcharger 34 (ACT 24). Furthermore, the peeling member 35 peels off thesheet P neutralized by the peeling charger 34 from the surface of thedrum 31 (ACT 25). The peeling member 35 guides the sheet P peeled offfrom the surface of the drum 31 to the sheet discharging unit 14. Thesheet discharging unit 14 discharges the sheet P, which is supplied fromthe drum 31 by the peeling member 35, to the sheet discharging table 54by the transport roller 51, 52, and 53 (ACT 26).

As mentioned above, the image forming apparatus forms the image on thesheet which is electrostatically attracted to the drum, and suitablysupplements the electric charge to the sheet attracted to the drum. As aresult, it is possible to supplement the electric charge which leaksfrom the sheet attracted to the drum along with the passage of time orthe number of revolutions, whereby it is possible to maintain theelectrostatic adhesion force of the sheet to the drum. As a consequence,according to the image forming apparatus, it is possible to realize ahigh quality image formation by the stable adhesion of the sheet to thedrum.

Furthermore, the image forming apparatus determines the amount of theelectric charge, which is supplemented to the sheet attracted to thedrum, depending on the printing conditions such as the printing rate,the color, the resolution, the thickness of the sheet, the transportspeed of the sheet, or the humidity in the apparatus (or the peripheryof the apparatus). As a result, the image forming apparatus can realizea stable adhesion force of the sheet onto the drum by the supplementaryamount of a suitable electric charge without an overload depending onthe printing condition.

Furthermore, the image forming apparatus supplements the electric chargeto the tip, the rear, or any of the tip and the rear in the sheetattracted to the drum. As a result, according to the image formingapparatus, it is possible to effectively supplement the electric chargeto the sheet without waste.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A recording medium holding apparatus comprising: an endless devicewhich rotates; a power supply unit which supplies a recording mediumonto the surface of the endless device with an electric charge; and anelectric charge supplementary unit which supplements the electric chargeto the recording medium which is rotated together with the endlessdevice in the state of being attracted to the surface of the endlessdevice by the electric charge provided by the power supply unit.
 2. Therecording medium holding apparatus according to claim 1, wherein, theelectric charge supplementary unit supplements the electric charge tothe recording medium whenever the endless device is rotated once.
 3. Therecording medium holding apparatus according to claim 1, wherein, theelectric charge supplementary unit supplements the electric charge byapplying an electric potential difference to the recording medium, theelectric potential difference being equal to or greater than an electricpotential difference that is applied by the power supply unit forproviding the recording medium the electric charge.
 4. The recordingmedium holding apparatus according to claim 1, wherein, the electriccharge supplementary unit supplements the electric charge to therecording medium attracted to the surface of the endless device in anon-contact manner.
 5. The recording medium holding apparatus accordingto claim 1, wherein, the electric charge supplementary unit neutralizesthe recording medium in case of peeling off the recording medium fromthe endless device.
 6. The recording medium holding apparatus accordingto claim 1, wherein, the electric charge supplementary unit neutralizesthe surface of the endless device after peeling off the recording mediumfrom the endless device.
 7. The recording medium holding apparatusaccording to claim 1, wherein, the electric charge supplementary unitsupplements the electric charge to a tip of the recording medium.
 8. Therecording medium holding apparatus according to claim 1, wherein, theelectric charge supplementary unit supplements the electric charge to arear of the recording medium.
 9. The recording medium holding apparatusaccording to claim 1, wherein, the electric charge supplementary unitsupplements the electric charge by applying an electric potentialdifference to the recording medium depending on the state of therecording medium.
 10. The recording medium holding apparatus accordingto claim 1, wherein, the endless device is a drum.
 11. An image formingapparatus comprising: an endless device which rotates; a power supplyunit which supplies a recording medium on a surface of the endlessdevice with an electric charge; an electric charge supplementary unitwhich supplements the electric charge to the recording medium which isrotated together with the endless device in a state of being attractedto the surface of the endless device by the electric charge provided bythe power supply unit; and an image forming unit which forms an image onthe recording medium attracted to the surface of the endless device. 12.The image forming apparatus according to claim 11, wherein, the electriccharge supplementary unit supplements the electric charge to therecording medium whenever the endless device is rotated once.
 13. Theimage forming apparatus according to claim 11, wherein, the electriccharge supplementary unit supplements the electric charge by applying anelectric potential difference to the recording medium, the electricpotential difference being equal to or greater than an electricpotential difference that is applied by the power supply unit forproviding the recording medium the electric charge.
 14. The imageforming apparatus according to claim 11, wherein, the endless device isa drum.
 15. The image forming apparatus according to claim 11, wherein,the electric charge supplementary unit supplements the electric chargeto a tip of the recording medium.
 16. The image forming apparatusaccording to claim 11, wherein, the electric charge supplementary unitsupplements the electric charge to a rear of the recording medium. 17.The image forming apparatus according to claim 11, wherein, the electriccharge supplementary unit supplements the electric charge by applying anelectric potential difference depending on the state of the recordingmedium to the recording medium.
 18. The image forming apparatusaccording to claim 11, wherein, the electric charge supplementary unitsupplements the electric charge by applying an electric potentialdifference to the recording medium depending on an image formingcondition of the recording medium.
 19. An image forming methodcomprising: rotating an endless device; supplying a recording medium ona surface of the endless device with an electric charge; supplementingthe electric charge to the recording medium which is rotated togetherwith the endless device in the state of being attracted to the surfaceof the endless device by the electric charge; and forming an image onthe recording medium attracted to the surface of the endless device. 20.The image forming method of according to claim 19, wherein the electriccharge is supplemented to the recording medium whenever the endlessdevice is rotated once.